The hazards of working in front of video display units were discussed in the 1980s. As a result of these discussions, it is now a standard requirement that emissions of various kinds, such as magnetic fields, electrical fields, etc, shall be kept to the lowest possible level. The discussions have resulted in methods for voluntarily testing video screens. As from 1987, there has been a strong commercial demand to fulfil certain recommended measurement values in conjunction with the recommended measuring method. In certain instances, it is necessary to fulfil still more stringent, customer-specific requirements. The ability to show comparatively low measurement values in these voluntary tests has given a competitive edge to retailers. The revised methods for voluntarily testing video display units have included testing for electrical alternating fields. These fields are often measured in two frequency bands, band I (5 Hz-2 kHz) and band II (2 kHz-400 kHz).
The electrical alternating fields deriving from a visual display unit, primarily a CRT type unit (cathode ray tube unit) are generated by a number of different electrical components included in the unit. It is a relatively easy matter to screen the electrical alternating fields in all directions with the exception of the forward direction immediately in front of the screen, for instance with the aid of screening plates, electrically conductive outer casings or housings, or with the aid of electrically conductive layers or coatings on a plastics outer casing. The electrical alternating field that extends forwardly from the screen surface of a CRT type video display unit is caused by variations in the acceleration voltage applied inwardly of the front glass of the video display unit.
One method of dealing with the effect of the electrical alternating fields that are generated in front of a display screen among other things, involves the use of a transformer in the high voltage part with low internal impedance, although this solution is expensive if the result is to be effective. Another method involves coupling an external capacitor across the acceleration voltage. This solution results in smoothing of the acceleration voltage.
Other methods involve the provision of an electrically conductive panel or like device in front of the picture tube. This solution requires a panel of a very high conductivity in order to be able to reduce the alternating fields in both band I and band II. Consequently, such panels are expensive and/or result in impaired synergonomical properties. For instance, the picture becomes out of focus, ie is blurred, and the light yield is inferior to the light yield of a standard screen, ie the panel does not have full light transmission.
An object of the present invention is to achieve with a video display unit, and primarily a CRT type unit, the smallest possible electrical alternating fields in the surroundings of the picture tube, and particularly in front of the screen of the unit.
To this end, there is described in the International Patent Application No PCT/SE92/00794 (WO-A1-9310537) which is assigned to the same Assignee as the present Application, an arrangement for reducing to a minimum the electrical alternating fields that are generated in the surroundings of a visual display unit. The display unit is provided with a voltage-connected or live part, eg an electrically conductive coating, on which undesirable signal variations occur. The undesirable voltage variations are indicated, a signal which is phase-reversed in relation to the indicated voltage variations is generated, and the signal is applied to the voltage-connected or live part. In the case of this known arrangement, the undesirable voltage variations are mainly detected indirectly, such as from the deflecting coil pack and across a winding on the high voltage transformer. This method has been adopted because the main object was to compensate for undesirable voltage variations within band II and because the main source of these variations is derived essentially as the result of the capacitance that is formed between the aluminium layer of the anode on the inside of the picture tube and the deflecting coil pack around the neck of said tube.
Another object of the present invention is to compensate for relatively low-frequency voltage ripple on the anode or the electrically conductive layer adjacent thereto of a CRT type visual display unit, particularly within band I.
A further object of the invention is to provide for different degrees of compensation for voltage ripple within band I and band II.